Linux
6 curated resources, easiest first.
Practical advice on avoiding self-inflicted breakage: do not mix releases, prefer packaged drivers, avoid make install and read APT's removal lists before confirming.
Explains how to install Flatpak on Debian, add the Flathub repository and install and run sandboxed apps, with notes on security trade-offs.
A short disambiguation page that explains the difference between the APT package system and the apt command-line tool, and points to deeper pages.
Explains how to update package lists, install, remove, purge and upgrade software with apt, plus pinning, mixing releases and cleaning up automatically installed packages.
How to control which repositories APT uses, including the newer deb822 .sources format, fields like Suites and Components, and migrating old sources.list files.
Official documentation for Ubuntu Server, including a section on managing software packages, updates and upgrades alongside installation, networking and security.